I recently worked on a handful of tracks where the bass guitar was recorded through a DI, with plans to reamp later in a better room with better equipment and a better engineer than myself. The DI signal was sent out to the amp and captured with a single mic, between 4' and 6' away. Now that I'm in the mixing stage, I'm noticing certain notes become much more full in the bottom end when I flip the phase of either the DI or the reamped track. But it's only certain notes that exhibit this behaviour. Does anyone else ever have this issue? I would guess that it maybe has something to do with the different wavelengths of each note, but I've never noticed such drastic frequency cancellation when I've combined DI and reamped signals in the past. It seems kind of insane to go through the track and flip a bunch of single notes to keep the low end consistent, but what do I know? Any thoughts on this?

It's likely that the signal isn't 180 degrees out of phase but somewhere in between. Flipping the polarity on one element just changes where the comb filter effect happens. There are plugs and hardware that will do variable phase shifting. They're designed for situation just like this. I don't have any so I cant recommend but I'm sure someone can and will.
If you're in digital try zooming way in on both wave forms (di and amp). Is one slightly delayed? Tube amps and DIs often have a different reaction time. Even more so if the input signal chain has a bunch of stuff going on. It's in the micro micro seconds but it can effect tone. If that's the case, what happens when you slide one signal to line up with the other? What happens when you do that and then reverse the polarity on one side?

A DI signal will be 'ahead' of your reamped-mic-captured signal as a point of mathematical certainty.

You should be able to visually align these by picking one of the loudest fastest transients in the take- just because the peak and valley should be quite obvious there, then slide the DI forward so it's hitting as 'late' as the mic signal, or visa versa.

You may find that the mic signal sounds great alone, but if you get the knack for correcting phase slippage in the box it will be such a fast fix you might as well figure out if the blend is yielding better results than the amp channel alone.

From there, I sometimes find that the phase relationship is generally good, but there's still some subtle interference. This can often be cleaned up more by assigning each signal a role that serves it best: usually the DI gets the role of the fundamental, and the mic is the overtones in the mids. I will sometimes aggressively low pass the DI, and high pass the mic, then tailor each eq to emphasize the growl of the mic and the oomph of the DI, if it needs more eq at all.

Edit: I did the thing where I reversed the terms 'high pass' and 'low pass'. Classic.

Last edited by losthighway on Tue Nov 06, 2018 5:52 pm, edited 1 time in total.

if the mic was 4 to 6 feet back in the room it's entirely possible that certain frequencies were getting boosted/cut due to phase interference in the room. if you listen to the reamped track alone, do those same notes jump out?

Assuming the answer is "yes", but you've confirmed that the notes in question are not jumping out or disappearing when only using one or the other right? Some basses just aren't set up properly and exhibit this quality. Check in headphones too and make sure there isn't just a null in your listening environment too.

EDIT: I initially read that as 4"-6". You mean FEET? that's bananas. Of course shit is cancelling out. If you want the mic'd tone to be as close as possible in arrival time to the DI, you need to place the mic A LOT closer.

4-6 feet of air delay means roughly 4-6 mS of time offset, which I think gets you at 180 degrees out of phase at somewhere around 200Hz, so you're going to see some lesser cancellation even lower. Throw in some converter latency and you definitely have a phase problem... which you won't fix using the polarity switch.

Since the mic was placed so far away I'll assume they/you were going for an "open" sound. In that case, delaying the mic signal even further could solve your phase problem and create a cool bass sound? You'd probably want more DI than mic to keep it sounding solid, however.

Another option would be to filter the low- or high-end out of one or both signals.

Or maybe just re-amp again! When I re-amp bass it's mostly for the cab/speaker tone, not the room itself.

If you're in digital try zooming way in on both wave forms (di and amp). Is one slightly delayed? If that's the case, what happens when you slide one signal to line up with the other? What happens when you do that and then reverse the polarity on one side?

Yes, the amped signal is delayed about 300 samples (6ms?) on this particular track. I lined them up best I could and that definitely brought back some fullness on some notes, but had the opposite effect on others. Flipping the polarity does what you'd expect in this case (full notes get thinner and and thicker notes thin out).

From there, I sometimes find that the phase relationship is generally good, but there's still some subtle interference. This can often be cleaned up more by assigning each signal a role that serves it best: usually the DI gets the role of the fundamental, and the mic is the overtones in the mids. I will sometimes aggressively low pass the DI, and high pass the mic, then tailor each eq to emphasize the growl of the mic and the oomph of the DI, if it needs more eq at all.

if the mic was 4 to 6 feet back in the room it's entirely possible that certain frequencies were getting boosted/cut due to phase interference in the room. if you listen to the reamped track alone, do those same notes jump out?

Both the reamped track and the DI sound ok on their own, listening through speakers or headphones.

Since the mic was placed so far away I'll assume they/you were going for an "open" sound. In that case, delaying the mic signal even further could solve your phase problem and create a cool bass sound? You'd probably want more DI than mic to keep it sounding solid, however.

Another option would be to filter the low- or high-end out of one or both signals.

Or maybe just re-amp again! When I re-amp bass it's mostly for the cab/speaker tone, not the room itself.

I bet the tone controls on the amp weren't set flat. Filters affect phase.

You'd probably do better limiting the frequency range of each of the tracks so that they don't overlap in the low end. A lot of people use the DI for the more open and dynamic low end and then highpass the amp track for the fun stuff it does in the mids and highs.